1. Field of the Invention
The present invention relates to flat cables, more particularly to a flat cable preferably for use in high-frequency signal transmission.
2. Description of the Related Art
As an invention relevant to a conventional flat cable, a high-frequency signal line described in, for example, International Publication WO 2012/073591 is known. The high-frequency signal line includes a dielectric element assembly, a signal line, and two ground conductors. The dielectric element assembly is formed by laminating a plurality of dielectric sheets. The signal line is provided in the dielectric element assembly. The two ground conductors are opposed to each other with respect to the signal line in the direction of lamination of the dielectric element assembly. Accordingly, the signal line and the two ground conductors form a stripline structure.
Furthermore, the ground conductors have a plurality of openings overlapping with the signal line when viewed in a plan view in the direction of lamination. As a result, less capacitance is created between the signal line and the ground conductors. Therefore, it is possible to shorten the distances between the signal line and the ground conductors in the direction of lamination, resulting in a thinner high-frequency signal line. Such a high-frequency signal line is used, for example, to connect two circuit boards.
Incidentally, the high-frequency signal line described in International Publication WO 2012/073591 might have defective connections between the circuit boards and the high-frequency signal line, as will be described below. FIG. 17A is a top view where the high-frequency signal line 500 described in International Publication WO 2012/073591 is bent in two places into a Z-like shape. FIG. 17B is a side view where the high-frequency signal line 500 described in International Publication WO 2012/073591 is bent in two places into a Z-like shape.
When the high-frequency signal line 500 is intended to connect two circuit boards, the length of the high-frequency signal line 500 is typically designed to be equal to the distance between external terminals (e.g., connectors or flat terminal electrodes) of the two circuit boards. However, in such a case, the high-frequency signal line 500 is stretched between the two circuit boards, generating a force to detach the high-frequency signal line 500 from the external terminals. That is, there might be defective connections between the circuit boards and the high-frequency signal line 500.
To solve such an issue, the length of the high-frequency signal line 500 is designed to be longer than the distance between the external terminals of the circuit boards. In addition, the high-frequency signal line 500 is bent in two places, as shown in FIGS. 17A and 17B. As a result, the length of the high-frequency signal line 500 can be adjusted properly so as not to generate a force to detach the high-frequency signal line 500 from the external terminals.
However, when the high-frequency signal line 500 is bent in two places, the high-frequency signal line 500 overlaps in three layers, as shown in FIG. 17B. Therefore, even if the high-frequency signal line 500 is reduced in thickness, it is thick at the overlapping portions of the three layers.